Shadow Map treats the depth map as if the image were rendered from a
light source. The
Ps parameter represents the position of the point
being shaded in the space of the light’s projection. The spread is a
softness control on the shadow, while the bias controls how accurate the
depth samples need to be. The quality is a general control to
increase/decrease sampling. A quality of 1 is typical quality.
The fractional amount of illumination which reaches the sample point will be returned. For example, if the point is fully in shadow, the return value will be 0, if it is fully illuminated, the return value will be 1.